common: modify md_config_obs_impl API

common: modify md_config_obs_impl API

Adding a new version of the API used by the configuration
manager to find out what keys are handled by the specific
observer (and thus should be registered to be updated on
changes).

The new interface (get_tracked_keys()) returns a vector
of moveable strings.

Modify the ObserverMgr to use this new interface
when avaiable. The existing 'char**' based interface
is maintained for now, until all its clients are updated.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>